It is known in the art to define the refractive index of a material by means of the critical angle of the total reflection. In the measuring method, the critical angle of the total reflection of light is measured at the interface of the optical window, so-called measuring window, and the substance being measured, such as liquid. The refractive index being measured is then obtained by means of the Snell lawn=ni sin αc.  (1)
In equation (1), n is the refractive index of the substance, such as liquid, being measured, ni is the refractive index of the material of the measuring window and αc is the critical angle of the total reflection. According to an exemplary embodiment, the refractive index of the measuring window should be bigger than the refractive index of the substance being measured.
It is also known in the field to have the critical angle of the total reflection express as a boundary of light and dark areas by directing the light reflected from the interface of the window and liquid by means of a lens system to a cell of an image detector, such as camera.
In known designs, the lens system can be set to be at a distance equal to its focal length from the camera. Refractometers, in which light is directed through prism-like surfaces to the interface of the liquid being measured and measuring window, are also known components. Prism-like surfaces are always oblique in relation to the interface.
Liquids to be measured in different processes can be pressurized. Therefore, the measuring window should be sealed to prevent the liquid being measured from entering the measuring device. To achieve good pressure resistance, the sealing can be done in such a manner that the pressure directed to the measuring window presses the seal against the frame part of the measuring device. Oblique prism-like surfaces used in measuring windows of known systems can complicate the sealing of the measuring window. For example, using a known oblique prism-like structure can make it difficult to obtain a structure in which the process pressure would press the seal to be tighter.
Many prism-like solutions based on surfaces that are oblique in relation to the measuring surface are known in the art. Examples of known solutions are described in publications U.S. Pat. No. 4,451,147, EP 0 359 167 A3, U.S. Pat. No. 3,628,867, GB 2014724, FI 65496, FI 96451, U.S. Pat. No. 6,760,098, EP 0 836 092, EP 0 281 337 and U.S. Pat. No. 4,962,746.